Chicken v Egg, which first? (question for Novosibir/others?)

neikeel · Post by **neikeel** » Sat Dec 09, 2006 11:14 am

Hi

I am slowly sorting out my 69 JMP 50 which was sold to me with a story of blowing fuses and not having been used for several years. Apart from a late 70s replacement Drake PT (1202-324) it is all original. The PT checks out according to the expected R measurements for all the leads.

Why do we hardly ever see one of these without a blown PT or replacement diodes or snubber caps?

What is the weak link?

I have replaced the filter caps with some of the last batch of LCRs and have acquired (but not replaced,yet) bias caps, snubber caps or diodes.

I would like to keep the amp as original as possible otherwise I would replace these all immediately. Externally they look ok. (The 10uFs are blue Hunts, the 0.22uF are white/blue Dubillier and the diodes are in the standard square block.

I plan to fire the amp up with your 100k filter cap reforming method you sufggested.

I wondered how I can test these components with my DMM (or AVO Mk VIII) in situ before fire up and particularly before I put my 'testing' set of tubes in?[/img]

neikeel · Post by **neikeel** » Wed Dec 13, 2006 10:56 am

Nobody know?

Perhaps I am not as ignorant as I thought

novosibir · Post by **novosibir** » Wed Dec 13, 2006 3:20 pm

neikeel wrote:I have replaced the filter caps with some of the last batch of LCRs and have acquired (but not replaced,yet) bias caps, snubber caps or diodes.

I would like to keep the amp as original as possible otherwise I would replace these all immediately. Externally they look ok. (The 10uFs are blue Hunts, the 0.22uF are white/blue Dubillier and the diodes are in the standard square block.

I plan to fire the amp up with your 100k filter cap reforming method you sufggested.

I wondered how I can test these components with my DMM (or AVO Mk VIII) in situ before fire up and particularly before I put my 'testing' set of tubes in?[/img]

Sorry! Right now first time saw your post!

With a DMM you can't measure caps, for this you'd need a LCR meter!

On DMM's sometimes is a position 'diode test', with this you might check the four diodes inside the block.

But don't put the tubes in the amp to form/reform the caps!

Larry

MarkCameron · Post by **MarkCameron** » Wed Dec 13, 2006 9:54 pm

I think the main thing that would make the tubes blow, is how the bias(wite wire) is connected ..after.. the Stand By (Its a Marshall boo boo). Follow the blue wire back to the switch and connect the wite wire to the hot side of the Stand By.

robert · Post by **robert** » Thu Dec 14, 2006 4:14 am

Hi Neikeel,

in the JMP50 this "Black Block" rectifier is not used as a bridge rectifier, but for two way rectification.
I argue that the peak-inverse-voltage (PIV) rating of the diodes inside the rectifier block is too low. Add two UF4007 in series to the diodes in the block to increase the PIV.
Or, at least, inatall two 500mA Slo-Blo fuses between the HT winding and the Standby switch.
Like Mark wrote, the bias wire has to be connected directly to the HT winding, and not behind of one of the the (two new added) fuses or even after the standby switch (cold side) like it was done by Marshall in the past.
The bias voltage must always "be there", absolutely independent of the standby switch position or eventually blown HT fuses!

Regards

Robert

neikeel · Post by **neikeel** » Thu Dec 14, 2006 6:49 am

Thanks Folks

That may explain a lot.

I will hook up the extra diodes, I always thought it was odd using a bridge type rectifier and only using half of it.

It also explains why my 76 JMP50 has the bias wire hooked up in just this way, it worked so I had not changed it.

I will mod my other amps too (no point in originality if it goes bang!)

robert · Post by **robert** » Thu Dec 14, 2006 7:30 am

+1!
But wait, maybe Larry comes up with a smarter solution than mine for this problem to keep at least "the look" authentic

Regards

Robert

neikeel · Post by **neikeel** » Thu Dec 14, 2006 7:34 am

Will do

novosibir · Post by **novosibir** » Thu Dec 14, 2006 9:41 am

No smarter solution! It's already said all about above

And yes, the most important thing was to move the bias supply wire to the hot side of the standby switch!

Larry

neikeel · Post by **neikeel** » Sun Dec 17, 2006 11:47 am

I have been looking at the spec of the diodes and can only assume that Marshall used those diode blocks because they got a load cheap

By my (admittedly dodgy calculations

) the PIV spec.for this application should be around 1200V, current is negligible. The diode blocks seem to be a high current rated item (I have usually seen them sitting on heat sinks before).

My block appears to be original (red blobs intact) but the original PT obviously 'went south'.

I may just replace the block with four diodes as the board is drilled for them unless I find a higher spec block. I am not htat precious about originality. Next stop the RS catalogue....

Flames1950 · Post by **Flames1950** » Sun Dec 17, 2006 1:37 pm

There are plenty of higher spec blocks out there, but none that I've found have the round post terminals, they all have big wide spades.

Metropoulos Forum

Chicken v Egg, which first? (question for Novosibir/others?)

Chicken v Egg, which first? (question for Novosibir/others?)

Re: Chicken v Egg, which first? (question for Novosibir/othe